Carotenoid compounds have a polyene chain structure, and specific examples of such include .beta.-carotene, tycopene, astaxanthin and the like. .beta.-carotene is known as pro-vitamin A, which decomposes to vitamin A according to the needs of a living body.
Carotenoid compounds are generally used as natural pigments for foodstuffs, and are apt to selectively react with carcinogens such as singlet oxygen radical and the like, and as such, they are expected to have use as a prophylactic agent for cancers. In light of this expectation, there is an increasing need to develope a process that can effectively and efficiently synthesize the polyene chain structure.
.beta.-carotene has been manufactured by Hoffmann-La Roche since 1954, and by BASF since 1972 [Paust, J., Pure Appl. Chem., 63:45-58 (1991)].
According to the Roche process, two C.sub.19 molecular units are connected by using bis(magnesium halide) acetylide, and the resulting product is subjected to partial hydrogenation of the triple bond and dehydration in the presence of acid catalyst, to provide .beta.-carotene, as shown in Scheme 1 below: ##STR2##
As can be seen from Scheme 1, however, the synthesis of the C.sub.19 compound from the C.sub.14 compound is not a convergent process, and requires two consecutive enol ether condensations, thereby providing the process with a low effectiveness.
With regard to the BASF process, .beta.-carotene is synthesized via a Wittig reaction of C.sub.15 phosphonium salt and C.sub.10 dialdehyde, as is shown in Scheme 2 below. According to this process, a double bond can be effectively formed by the Wittig reaction, but the process has a further problem in that phosphine oxide (Ph.sub.3 P.dbd.O), produced as a by-product, cannot be easily separated or removed. ##STR3##